This section provides background information related to the present disclosure that is not necessarily prior art.
It is challenging to prepare high quality class-A surfaces using thermoset composite parts such as sheet molding compound (SMC) panels. SMC panels are typically joined to a support structure using thermoset adhesives prior to attaching them to vehicle bodies. For example, exterior SMC door panels may be bonded to an internal reinforcing member using a thermoset adhesive. Typically, the thermoset adhesives are cured with thermal cure cycle to produce a strong joint. However, the thermal cure cycle intended to cure the adhesive introduces minute surface distortions in the vicinity of the bond-line known as bond-line read-out (BLRO), which deteriorates the visual appeal of the class-A surface. Significant BLRO often results in part rejection and the subsequent time and cost of rework.
In the automotive field, a thermoset adhesive may be cured using a two-stage process. In the first stage, uncured adhesive is applied between the substrates to be joined. External pressure is then applied upon a bonding fixture to maintain dimensional tolerances as it is heated at the cure temperature for 3-5 minutes. Heat may be supplied to the adhesive until the adhesive is cured to “green strength,” which allows one to handle panels joined by adhesive before the adhesive is completely cured. “Green strength” allows joined panels to continue through assembly while the adhesive continues to cure. While the panels and adhesive are heated, the panels experience an overall rise in temperature that results in thermal expansion. This, in combination with any differences in the coefficients of linear thermal expansion (CLTEs) between the panels, also leads to distortion of the bond-line. At the same time, the adhesive itself marginally distorts due to chemical cure shrinkage. As the adhesive cures partially to green strength, a fraction of these distortions are retained in the joint. The panels joined to green strength are then painted in a paint booth followed by additional heating to cure the paint. The paint “bake” cycle requires another 20-30 minutes of heating of the joined panels, resulting in further surface distortion.
Curing thermosetting adhesives with heat produces surface distortion or BLRO due to thermal expansion induced dimensional changes in the SMC material and cure shrinkage in the adhesive cure. Thicker SMC panels can be employed to increase stiffness and keep distortions below a threshold at which they are perceptible to the unaided eye. Using panels that are thicker than necessary for such structural requirements, however, adds weight and undermines the reason—weight reduction—for using SMC panels in the first place in automotive applications.
The visual quality of automotive exterior, class-A surfaces for SMC panels may also be affected by cure shrinkage in the SMC itself during curing of molded SMC composition. Inter-fiber micro-shrinkage in SMC causes fiber read-out (FRO) defects in the surface of the SMC, which can be described as fibers revealed on the surface of the SMC. Such surface defects of the SMC are magnified during the surface finishing steps, telegraphing the defect onto the surface of the paint finish. Low-profile additives now added to SMC to prevent or minimize FRO themselves create voids that adversely affect the physical properties of the SMC. In addition, during subsequent coating bakes in which applied coating layers are thermally cured, the voids outgas to cause paint-pop defects in the cured coating layer.
These problems of BLRO, FRO, and paint-pop defects in the finish have been significant impediments to using SMCs in automotive class-A surfaces.